[2.x] Fix ignore generic listeners as view composer

[arjanwestdorp]

This fixes #793 which would throw a ReflectionException when a non-closure listener is attached to the composing: ..., creating: ... or * event.
This happens when a listener is attached like: Event::listen('*', 'MyListener@handle')

All view composers/creators are wrapped in a closure, see: Illuminate\View\Concerns\ManagesEvents@addViewEvent. That means that we can filter out all listeners that are not a closure when fetching the view composers.

Technically it would be possible though to create a view composer like Event::listen('composing: layouts.master', 'MyListener@compose'). But since there is a dedicated View::composer method I assume that's the prefered and supported way to register view composers? If we would add support for those non-closure listeners that would also mean that all listeners for the * event will be seen as view composer.